The present invention relates generally to piling and pile driving and more particularly to an improved piling connector and a unique method of connecting pilings. It is equally adaptable to both new construction and for repairing existing construction, and has particular utility in the repair of concrete slab foundations on pilings. Similarly, it is applicable in all terrain conditions in which pilings are used but has particular utility in the most difficult conditions such as saturated soils and terrains in which the water content exceeds complete saturation.
The problems which the present invention overcomes are long-standing and have been known for decades if not longer. For example, friction pilings of the type and size for which this invention is expected to be frequently employed typically might have a ten ton maximum rating. That is to say, such pilings typically are not able to resist a total downward force in excess of twenty thousand pounds. If the environment in which such pilings are to be used is expected to produce a downdrag force of two tons, the pilings may be analyzed as consisting of a two-ton downdrag portion and only an eight-ton frictional resistance portion. The conservative designer will then subtract the two tons of downdrag force from the eight tons of frictional resistive force to obtain a net maximum of six tons per piling and then, in order to have a reasonable margin of safety, use one-half that number, or three tons, as the design capacity of such pilings. Knowing the maximum load which the particular foundation must support, the designer would then calculate the number of pilings needed and distribute that number about the foundation.
The difficulty with super-saturated soils, and even in many soils that are less than saturated but near saturation, is that such soils usually will not remain uniformly wet. When dry, or even when only partially dry, such soils experience enormous contractions, and as they settle, extremely strong downward forces are created. When the downdrag exceeds the maximum resistive force of the friction pilings, failure results.
Due to the difficulty of access, repair of such a failed foundation is typically quite expensive. For reasons of economy, most friction pilings are wooden poles or, literally, de-barked trees. To prevent decay, and subsequent foundation failure as a result therefrom, wooden pilings are commonly treated with preservatives. However, full-length treated pilings typically cost from twice as much as untreated pilings of the same length and diameter, up to three times as much.
Generally, the deeper a piling is set, the greater is its capacity to resist downward forces. In fact, it is not at all uncommon for the resistive force or resistive capacity of such pilings to increase in a non-linear manner with depth. A typical soil profile in which pilings are normally used may provide three tons of resistive capacity at thirty feet of piling length, four tons at forty feet, but perhaps eight tons at sixty feet. Thus it is apparent that the deeper the designer places the pilings, the greater the capacity, perhaps non-linearly greater, and the fewer the number of pilings needed. Offsetting this advantage, however, is the fact that the longer the one-piece piling, the greater is the costxe2x80x94also a non-linear function. If the installed cost of a treated thirty-foot residental or light commercial piling (e.g., a Modified Class Five piling) in a particular locale is fifty-two dollars, for example, the cost for a forty-foot piling might be seventy-five dollars, and the nearest comparable sixty-foot piling, three hundred thirty dollars.
The dramatic increase in costs for exceeding forty feet is due to several factors, one of which is that the piling material itself must be of a larger class in order to achieve the desired length; this necessitates a non-linear increase in the cost of the material employed. In addition, small xe2x80x9chouse rigsxe2x80x9d can be used to drive pilings up to forty feet; the costs for driving piles with such equipment is typically as low as fifty cents per foot. Going beyond the 40-foot limit, however, exceeds the capacity of such small equipment; much larger driving rigs must be used, the cost of which may be as much as five dollars per foot. Combined with the non-linear cost-of-material increase, the final, installed cost of a sixty-foot piling might typically be as much as four or five times the final, installed cost of a forty-foot piling.
The cost for treating extra-long pilings also increases non-linearly because of the more expensive equipment needed to treat such pilings. It is known that a piling need not be treated along its entire length in order to preserve it; only the portion above the lowest water table need be treated. However, since most treatment means call for the preservatives to be forced into the wood pores under high pressure, and since the non-uniformity of the raw materials makes consistent sealing around the circumference of the work pieces difficult to achieve, equipment which will pressure-treat only an end of a piling is typically either not available or so expensive as to not afford any savings.
The prior art has therefore looked to various means of connecting shorter pilings, i.e., each of forty feet or less, so as to make an effective and economically affordable longer piling. One such early attempt is that of U.S. Pat. No. 1,073,614, xe2x80x9cPile Splicexe2x80x9d, to W. A. McDearmid. McDearmid employs a specially-cast tubular body with an integral transverse partition dividing the body into two chambers of equal diameters. The device is placed over a snugly fitting lower pile, a short pin is driven longitudinally into the lower pile with one end protruding, the upper pile is then dropped into the upper chamber onto the pin, and a bolt is then passed horizontally through each chamber and secured by a nut on the distal end thereof. Several disadvantages are presented by this approach, however. One such disadvantage, if the holes in the pilings are pre-drilled, is the difficulty of precisely aligning the holes in the environment intended, i.e., under water or in semi-watery mud. If the holes are not pre-drilled in the pilings, it is virtually certain that a bolt secured through the piling in that environment would often not meet the opposite hole in the chamber.
Perhaps a greater disadvantage of the McDearmid splice, however, is the necessity to adapt or pre-prepare the ends of the pilings to be received in the connector. Not only is this step an additional expense, but if the pilings do not fit quite snugly within both chambers, there will be a tendency for the splice to act not like a rigid connection but pin-like about one or both horizontal bolts until further rotation is prohibited by the walls of the chambers. At this point an eccentricityxe2x80x94perhaps a destabilizing eccentricityxe2x80x94will already have been introduced into the system. The amount of resistance which the small, vertical pin would provide to such a moment is expected to be negligible.
Another approach is that of U.S. Pat. No. 4,525,102, xe2x80x9cTimber Pile Connection Systemxe2x80x9d, to Gerard J. Gillen, which also discloses a number of other prior approaches to this problem. Gillen appears to call for a hollow splice to be driven internal to each piling with a confined levelling material therebetween to avoid point or edge stresses and to distribute the forces at the interface more widely. Such an internal splice is of course at least partially destructive of the piling material. In addition, the piling itself becomes the xe2x80x9cweakest linkxe2x80x9d in that only a small fraction of the piling material remains exterior to the splice to hold the splice in place. A small error in aligning the splice along the longitudinal axis could easily cause failure during subsequent driving.
Further, it is apparent that the technique of Gillen will not produce a rigid mechanical joint. The joint will be held together only by the force of friction between a piling end and the connector, and once that resistive force is exceeded, the joint will be expected to come apart. This is equally true whether the disrupting force is due to a moment about the joint or to an in-line force applied during driving. The Gillen technique may be expected to xe2x80x9cdrive offxe2x80x9d the lower pile from time to time during routine pile driving, and to buckle the joint if a more resistive formation such as sand should be encountered.
Swedish Patent 85,932 discloses the use of a suitable number of randomly placed flat bars or straps over the joint between two pilings secured by nails. An internal dowel pin, comprising a central collar portion and a tapered pin portion protruding into each end of the pilings, is apparently relied upon for rigidity. The flat bars are intended to prevent the joint from being pulled apart, but they would not be expected to be able to resist any but small bending moments.
U.S. Pat. No. 4,696,605, xe2x80x9cComposite Reinforced Concrete And Timber Pile Section And Method Of Installationxe2x80x9d, also to Gillen, employs a means of connecting which apparently relies upon the rigidity of the concrete pile itself to maintain a rigid joint. While technically sound, such a method may often be economically impractical.
U.S. Pat. No. 3,266,255, xe2x80x9cDrive-Fit Transition Sleevexe2x80x9d, to Dougherty, employs a pair of flanged pipes telescoped one inside the other and force fitted to each other, much like a plug-and-socket arrangement. Dougherty, however, is obviously limited to connecting metal pilings, and calls for connecting the separate pieces of his connectors to the pilings by welding.
The present invention involves an improved piling connector which can transfer a bending moment and direct forces across a joint of a composite pile and a unique method of driving composite piles. Unlike pin-type connectors, or connectors which function essentially like a pin-type connector, the connector of the present invention will not allow one pile of a composite pile system to rotate with respect to the other or to induce an eccentricity into the overall, combined column. Further, a lower pile of this system may not be xe2x80x9cdriven offxe2x80x9d the joint while driving the pile assembly, and the connector may be chosen such that it will not be the weakest link in the assembly. Still further, no special preparation or sizing of the ends of the pilings must be done in order to employ the present invention.
A preferred embodiment of the improved connector of the present invention comprises two rigid tubular members joined by a rigid ring or plate with at least one opening permitting fluid communication between the tubular members. Each tubular member preferably has a plurality of holes in the wall thereof, spaced apart both circumferentially and longitudinally, with a deflector attached to the outer wall in alignment with and spaced apart from each hole. When employing one preferred method, a pile is driven in the customary manner until the upper end is at a convenient height above ground level. The battered end is sawn off, as is customary when driving wooden piles. An open end of the connector is then placed on the upper end of the piling, and is rapidly driven onto the piling by the driver hammer. Outer portions of the piling are peeled off by the connector as it is being driven onto the piling, and such peelings are deflected away from the holes in the wall of the connector by the deflectors. The connection is then made rigid, preferably by screwing lag screws, of a size sufficient to permit the transfer of forces between piling and connector, through the wall openings and into the piling. An end of the second piling is then positioned above the upper end of the connector, and that piling is driven into the connector and similarly made rigid, at which point the driving of the composite pile assembly may recommence. If desired, the lag screws may be inserted into both ends of the connector simultaneously.
It is to be noted that no preparatory work has to be done on the piling ends to prepare them for insertion into the connector. Rather, the chambers of the connector are selected so as to accomodate the particular piling ends. While it is preferable to size such chambers so that no voids will exist between the connector walls and the piling, it is not essential to do so inasmuch as the lag screws may be installed in such a manner as to resist bending moments also.
Piling systems of the type contemplated herein are also capable of resisting considerable forces in tension.
A variation of this technique has been found preferable for joining wooden and metal pilings, as in the repair of existing foundations where space for working is extremely limited. This techique is often useful where too few pilings have been employed, where too short pilings have been employed, or where the upper ends of the pilings have dry rotted or are not connected to the foundation they were intended to support. In a preferred application of this technique, a small excavation is made to expose the upper end of the piling to be extended or repaired or to be connected to the foundation. The connector is then positioned on top of such piling and forced into snug engagement therewith, preferably by hydraulic ram. This portion of the connection may then be made rigid as described above. Short sections of wooden or metal pilings may then be employed, sequentially as necessary, until the desired depth is reached or the desired resistance is encountered If metal members have been used, they may be left in place as is, if desired, or a continuous concrete column may be created by pouring cement therein.
In still another variation, new pilings may be driven under an existing foundation by employing a succession of short pilings.